A 54-year-old woman presented with a neck mass that encroached on the thyroid gland. An excision was performed of the mass and attached thyroid tissue. On microscopic examination, the mass abutted the thyroid but did not appear to arise from it. The images from the mass are shown below.
The mass consisted of somewhat irregular aggregates and nodules of neoplastic cells with often prominent central necrosis. A mixed inflammatory infiltrate was present between individual tumor cells and was more prominent between the tumor nodules. The neoplastic cells had prominent, somewhat "glassy" eosinophilic cytoplasm and enlarged vesicular nuclei with prominent nucleoli. In many cells the nuclei were eccentrically located in the prominent cytoplasm.
Immunohistochemical stains for cytokeratin, EMA and vimentin were positive. Melanocytic, hematopoietic, and dendritic cell markers were negative, as was CD5.
What is your diagnosis?
This is a difficult case. Many of the cells have a distinctly rhabdoid phenotype which led to the performance of an INI1 immunohistochemical stain. The results are shown below.
The tumor cell nuclei are completely negative for INI1 expression and the surrounding inflammatory cells form a nice positive control. This is an example of a proximal-type epithelioid sarcoma. Epithelioid sarcomas (proximal and distal), as well as rhabdoid tumors, are noted for their relatively specific loss of INI1 expression. In fact, the relationship between so-called malignant extra-renal rhabdoid tumor (MERT) and proximal type epithelioid sarcoma is controversial and overlapping, and these may well be two different faces of the same neoplasm.
The primary differential diagnostic considerations include carcinoma, melanoma, epithelioid sarcoma, and epithelioid angiosarcoma. Immunohistochemical studies will easily separate these entities, provided one is not misled by the cytokeratin positivity that may be seen in both epithelioid sarcomas and epithelioid angiosarcomas. CD31, and other vascular markers, are particularly helpful in recognizing angiosarcoma, and INI1 loss is specific in this context for epithelioid sarcoma.
The prognosis for any epitheloid sarcoma is guarded and the proximal-type lesions are especially aggressive. Surgical resection is the primary mode of therapy.